The metabotropic glutamate receptors (mGlus) are family C G protein coupled receptors (GPCRs) that function as constitutive dimers. These receptors are grouped into three main subfamilies and, until recently, mGlus were thought to assemble strictly as homodimers. Recently, however, we and others have shown that mGlus can assemble and function in a heteromeric fashion, and we have found that heteromeric complexes exist both in vitro and in native tissues. Additionally, the pharmacology of ligands interacting with homomeric or heteromeric assemblies show distinct profiles, and we propose that heterodimerization may underlie the unexpected pharmacological features of some ligands observed in vivo. We have shown that an mGlu2/4 complex, consistent with a heterodimer of these two protomers, is present presynaptically at corticostriatal synapses. Our preliminary data demonstrate that small molecule positive allosteric modulators (PAMs) of mGlu4 exhibit distinct profiles at various synapses where we propose mGlu4 exists as a putative homodimer (striatopallidal synapses) or as an mGlu2/4 heterodimer (corticostriatal synapses). These studies have shown that currently available mGlu4 PAMs exhibit two distinct profiles: compounds that potentiate both mGlu4 homomers and mGlu2/4 heteromers, and a second class that potentiates mGlu4 homomers alone. These findings are particularly interesting in light of studies suggesting that some mGlu4 PAMs, which also activate mGlu2/4 heteromers, exhibit activity in preclinical models of psychosis and anxiety, effects historically associated with mGlu2 activation. In contrast, PAMs that do not activate the mGlu2/4 heteromer do not show efficacy in these models. These studies are intriguing, yet still difficult to currently interpret due to the lck of a compound that potentiates ONLY the mGlu2/4 heteromer without activity at mGlu2 or mGlu4 homomers. Such a tool would be critical for interpreting in vivo activity of the mGlu2/4 heteromer and provide a highly significant advance for the field. We have now screened our internal mGlu4 PAM collection (~2,000 mGlu4 PAMs) for activity in cells expressing mGlu4 alone, mGlu2 alone, or cells co-expressing mGlu2 and mGlu4 and have identified numerous compounds with activity at the mGlu2/4 complex; these newly identified hits will serve as the basis of a chemical optimization program. We also have cell lines and assays in places to ensure selectivity of ligands for mGlu2/4 heteromers among the mGlus. Using recordings at both striatopallidal (mGlu4 homomer) and corticostriatal (mGlu2/4 heteromer) synapses, we will verify selectivity for the mGlu2/4 complex in native tissues. Finally, we will move into two in viv models that we believe represent activity of the mGlu4 homomer (haloperidol-induced catalepsy) and mGlu2/4 heteromer (marble burying). Overall, we anticipate that these studies will result in the development of the first highly optimized and selective mGlu2/4 ligand(s).